Nearly all fission reactions with the uranium U.sub.235 isotope operate with a greater concentration of that isotope than is typically found in the natural or depleted condition of the uranium element. Enrichment processes are typically employed to increase the proportion of the fissionable U.sub.235 isotope to the other isotopes in uranium, primarily U.sub.238. Enrichment is typically achieved through separation processes which operate on the slight chemical or physical difference between the U.sub.235 and the U.sub.238 isotope. Because these differences are slight, the enrichment which is achieved through a single separation step is typically very slight, and acccordingly the separation step is repeated many times in cascaded sequence to ultimately achieve the necessary enrichment.
As shown in our U.S. Pat. now Pat. No. 3,772,519, incorporated herein by reference, a promising new technique for isotope separation and particularly for uranium enrichment operates by selective excitation of energy states of one uranium isotope in an environment containing plural uranium isotopes. In typical application, a uranium vapor is created and illuminated with laser radiant energy having photon energies which selectively excite and ultimately ionize the U.sub.235 isotope without corresponding excitation and ionization of other uranium isotopes, chiefly U.sub.238. In achieving a significantly increased proportion of the U.sub.235 isotope in the ionized state than in the non-ionized state the laser radiation, and, in particular, the radiation which achieves the first excitation of U.sub.235 particles, is chosen to have a very narrow range of photon energies which corresponds to a U.sub.235 absorption line, but not a U.sub.238 absorption line. It is, therefore, possible to excite a substantial quantity of the U.sub.235 isotope in the uranium vapor without exciting the U.sub.238 isotope.
The excited and ultimately ionized U.sub.235 isotope is then typically accelerated out of the uranium vapor by the application of energy in one of several forms to the ions causing them to assume trajectories which will carry them toward collecting plates. The un-ionized uranium in the uranium vapor, consisting chiefly of U.sub.238 and whatever U.sub.235 has not been ionized and collected, continues on its normal path towards a further collecting surface.
Efficient separation and collection of the U.sub.235 isotope requires that in the initial excitation step all, or nearly all, of the U.sub.235 isotope in the uranium vapor is excited to an intermediate energy state. Such high efficiency further demands that an excitation radiation frequency be selected which will elevate U.sub.235 particles in energy from a beginning energy state to a second real energy state. The actual radiation frequency is defined by the difference in energy between those two states. For this, the initial state for those particles must be known.
One may assume that substantial numbers of the particles exist in the ground state and accordingly selectively excite those particles to an intermediate level using a corresponding laser frequency. However, as has been discovered, characteristics of the enrichment process result in significant populations of the U.sub.235 isotope in the uranium vapor at energy levels other than the ground or zero energy level. The excitation radiation will then, in general, be ineffective to excite that population of the U.sub.235 isotope which is not in the ground state, and accordingly it will not be separated along with the U.sub.235 particles originally in the ground state.
The presence of a significant population of U.sub.235 particles above the ground energy state, before application of the excitation radiation, may be explained as resulting from a random or particular distribution of energy in the uranium vapor. The existence of this energy in the uranium vapor may be attributable to the substantial quantities of thermal energy imparted to the uranium to vaporize it and to maintain its gaseous state, to particle collisions including interaction with energy beams, or other phenomena which result from the particular environment created in the application of selective excitation to produce enrichment.